The present invention relates to a dental instrument comprising an X-ray film support connected to a bite block and, preferably, also an aligning rod. In prior art instruments of the type above defined the bite block is homogeneous and its top surface is parallel to its bottom. When a patient opens his mouth, the lower jaw swings around the jaw-joints. This means that an arbitrary point on the lower jaw substantially moves along a circular arc the radius of which equals the distance of the point from an axis through the jaw-joints. Consequently, the farther back the bite block has to be located, the greater will be the aperture angle of the jaws. It follows that, when a bite block has been inserted between the upper and lower tooth rows of a patient and the latter has, obeying the instructions of his dentist, again closed his mouth, only the rearmost teeth will contact the bite block whereas a vertical distance will be left between the block and the upper or lower teeth at the front edge of the block. The lack of contact between the bite block and all teeth overlying it means that it assumes an unstable position and may easily move in response to variations of the bite pressure exerted by a patient. Such movements lead to a displacement of the film support and hence of the film. As is understood, such a swinging movement of the bite block can occur both around an axis parallel to that of the jaw-joints and along a horizontal axis parallel thereto.
The instability problem above referred to is aggravated in the following two situations. The one situation is when the rearmost molars are to be X-rayed. In order for the bite block to be received between the teeth the patient then has to open his mouth that much that the only contact between the teeth and the bite block will be at the rear edge of the latter. The second situation is when one or more teeth are missing, so that even when the frontal teeth are to be X-rayed the bite block does not get sufficient support.
The X-raying of the frontal teeth involves a further difficulty. Due to the anatomy of the palate, the film, and consequently also the film support, will be pressed downwards which often leads to exclusion from the picture of the tissue regions around the teeth roots. When the frontal teeth of the lower jaw are X-rayed the tongue of the patient causes corresponding problems. For that reason conventional film supports can normally not be used in these regions.
The main object of the invention is to provide a dental instrument in which the problems above referred to are eliminated, meaning that once the patient has pressed his teeth against the bite block the latter will securely be retained in the position selected by the dentist.
Other objects and advantages of the invention will appear from the description and the drawings which illustrate three different embodiments.